1. Field of the Invention
This invention relates to an impact wrench for the tightening of bolts using track rail by means of a plate-shaped tightening spring and to a method for tightening bolts using an impact wrench designed so that bolt tightening is achieved with the required spring compression force by measuring the angle of rotation of the impact shaft from the time at which the pre-set snug torque is being generated.
2. Description of the Prior Art
Track rails are secured by tightening bolts on to wooden and PC crossties by means of a plate-shaped tightening spring to hold down the rail. To tighten these bolts, a significant level of skill had been necessary as the application of the required tightening torque for tightening the bolts with the familiar impact wrench by means of the prescribed spring compression force had to be left to the judgment and feeling of the operator.
To achieve automatic control of the conventional type of impact wrench, the inventor has achieved further progress with the invention of an impact wrench using the rotating angle method in such a manner the angle of rotation of the impact wrench is measured and the electric motor is stopped when the predetermined angle of rotation has been reached. (Japanese Utility Model Registration Application No. 4430/1992).
The familiar impact wrench is designed so that a hammer is coaxially meshed with the impact shaft to rotate the bolt-tightening socket, with a force being applied to said hammer by means of a spring in the direction of the impact shaft. When bolts are tightened, the hammer rotates under the drive force of the electric motor and said impact shaft rotates while the hammer and the impact shaft are in mesh. When the bolt-tightening reaction force has become larger than the spring force applied to the hammer, however, the hammer will be lifted and separated from the impact shaft to permit its free rotation. Immediately after this, the hammer is again subjected to the spring's compression force to come into mesh with the impact shaft. As this mesh contact is obtained, a knocking force is applied to the impact shaft while the hammer is rotating so as to tighten the bolts.
The impact wrench thus requires a specific timing at which the measurement of the angle of rotation is commenced in order to ensure that the bolts are tightened by the fixed angle of rotation previously set by the rotational angle method.
The impact wrench based on the rotational angle method disclosed in Japanese Utility Model Registration Application No. 4430/1992, however, had been designed so that the timing for the release of the hammer from the impact shaft was specified in terms of the time at which the snug torque is generated so that the motor was stopped when the angle of rotation measured thereafter reached the predetermined amount of bolt-tightening.
In practical bolt-tightening operation, however, it happens that the bottom of the rail is lifted up without making contact with the crosstie (floating crosstie) before the bolt is tightened. The problem in such cases was that the possibility existed that the motor might be stopped before the correct bolt-tightening condition was achieved since, for the tightening of bolts by the above rotational angle method, the compression force for bringing the rail into contact with the PC crosstie was used as the reaction force acting in the upward direction so that the measurement of the angle of rotation would commence before the correct snug torque was detected.
In view of these earlier problems, according to the present invention a bolt-tightening method which is a combination of a torque method with the rotational angle method for automatic impact wrench control has been adopted. (Japanese Patent Application No. 19650/1993).
In this bolt-tightening method, the angle of rotation of the impact shaft and the torque are measured from the time at which the snug torque is generated so as to ensure that the electric motor will stop when both the impact shaft's angle of rotation and the torque have reached the prescribed values.
This type of bolt-tightening method is thus capable of resolving and overcoming the problem associated with the rotational angle method and the problem inherent in the torque method, that is, the problem of the electric motor's stopping before the tightening of the bolt is completed in the floating crosstie condition and the problem of variations in the tightening force applied to the bolt due to the influence of the conditions of the screw surface.
With this combined bolt-tightening method, however, the time at which the snug torque is generated was specified as the time at which the hammer is released from the impact shaft and impact is generated. As a result, a difference occurred in the time of completion of bolt-tightening by the rotational angle method and that by the torque method, thereby giving rise to the problem of errors arising in either of these methods.